In the current 3GPP (3rd Generation Partnership Project) architecture a P-CSCF (Proxy-Call State Control Function) is provided by the visited network, where the subscriber is roaming to in both Transport level and IMS level. FIG. 1 shows a P-CSCF in a visited network according to the current 3GPP architecture. This local P-CSCF has connection to the visited Transport Network as shown in FIG. 2. FIG. 2 illustrates a visited IMS built on top of the visited Transport Network represented by GPRS (General Packet Radio Service). Assigning/locating this local P-CSCF is the task of the visited network. For this purpose, two methods are defined in 3GPP:                0. DHCP (Dynamic Host Configuration Protocol) is used to provide a UE (User Equipment) with the domain name of a P-CSCF and the address of a DNS (Domain Name Server) that is capable of resolving the P-CSCF address.        0. A P-CSCF address is transferred within the PDP (Packet Data Protocol) Context Activation signaling to the UE.        
In other words, the current P-CSCF discovery methods only work when the visited IMS network comprises a local P-CSCF.
A more detailed description of the current 3GPP architecture is given in [TS 23.228 v5.00 (April 2001)].
However in the early stage of IMS deployment or in case of Alternative Access (i.e. Access Network other than GPRS) there may be the case that IMS subscribers roam into a Transport Network which has no IMS deployed on top of it and so the visited Transport Network cannot provide means to discover a local P-CSCF. Thus, the subscriber cannot access IMS for getting services therefrom.
Not providing IMS services for this kind of subscriber is the easiest way of handling the situation but surely is a great restriction on the business opportunity especially in the early phase of IMS deployment.